1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly to a zoom lens system suitable for a silver halide film camera, an electronic recording type digital still camera, an electronic recording type video camera, or the like.
2. Related Background Art
A so-called negative lead type zoom lens, in which a lens unit having a negative refractive power, is disposed on its front side has been used for a wide angle zoom lens in many cases, since it has characteristics of: “a distance for close-up image pickup is relatively short”, “a field angle can be relatively easily widened”, “a back focus can be relatively easily lengthened”, and the like.
On the other hand, in the negative lead type zoom lens, at a telephoto end, a first and a second lens units consist a positive group and a third and a fourth lens units consist a negative group, so that the entire optical system can be used as a so-called telephoto type. Therefore, there is a merit in that a focal length can easily be lengthened even on the telephoto end.
A zoom lens is disclosed in, for example, Japanese Patent Application Laid-open Nos. S57-011315, S58-095315, S59-229517, S60-055313, S60-087312, S61-062013, S61-123811, S62-063909, H02-136812, H04-235515, H04-163415 (corresponding to U.S. Pat. No. 5,132,848 B), H05-019170 (corresponding to U.S. Pat. No. 5,264,965 B), H05-313065, H06-082698, H07-287168 (corresponding to U.S. Pat. No. 5,585,970 B), Japanese Patent Application No. 2000-338397, and JP 2629904 B (specification). The zoom lens includes four lens units having negative, positive, negative, and positive refractive powers, in order from the object side to the image side. Zooming is performed by moving at least two lens units of these four lens units.
A zoom lens for single-lens reflex digital camera requires a further improvement in image quality as compared with conventional zoom lenses for silver halide film camera.
In general, when the refractive power of each of the lens units is increased in a zoom lens, the moving amount of each of the lens units necessary for obtaining a predetermined zoom ratio reduces. Therefore, it is possible to widen the field angle while the length of the entire lens system is shortened. However, when the refractive power of each of the lens units is merely increased, a variation in aberration with zooming becomes larger. In particular, when a wide field angle is to be obtained, it is hard to obtain preferable optical performance over the entire zoom range.
When an ultra wide field angle is set, it is hard to correct astigmatism. Therefore, there has been such a tendency that high performance cannot be obtained or a size of the zoom lens increases.